Hate the things, pa schematic out there but not prea it would seem. Preamp problem t'would seem. At switch on a pair of 47uF electros fed via 120K from
27V rail give increasing voltage that at about 8V , after 4 seconds, ends up putting a 3 to 5V pulse on the preamp outputs , straight into the power amp, both channels. This timed hold-off would seem deliberate, presumably muting off, but the DC surge?. Anyway this of course upsets the amp which then has to re-settle, otherwise seetles in first second , but now outside its own time-off period and delays relay turn on ,or worse , makes the relay chatter , via its DC loudspeaker protection cct activating. What to look for that the preamps , both channels oddly, come out of mute with a surge ?
I will look into that. The eservice schema, cannot even see whether FETs anywhere in the C version As 2 separate "timers" I will add another 120K to one and see if the effect is the same 2 sec in and 4 sec in. As it stands they time in near enough together but only one may be at fault and cross coupled into the other channel, otherwise 2 separate but identical faults seems iffy. It may be a hf oscillation problem . When I was scoping the amp output there was short bursts of 150KHz or so at this 4 second point. Did not think of scoping the preamp then to see if the hf was there also or just the DC pulse.
Yes, the timing certainly suggests this. At the end of the muting cycle, a large DC bias is applied to the base or gate. With the drain or collector sitting across the output to ground, this leaky transistor will then pass DC to the output. It has been a relatively common failure on Pioneer and Yamaha receivers, and I've seen it on Hafler preamps.
not had time to power it up again , but no FETs in there BC550 and BC560 only.
Is there any recognised circuit supplemental to allow for the replacement transistors going leaky over time? I suppose 20 years before metalisation creep/migration re-occurs in the replacement, is fair use.
DC pulse in the prea , not oscillation. Both channels the same , by changing one to 2 sec timing in. Using the eservice 8000C preamp schematic (bottom right splodge) as just litterally a pattern and then following traces and filling in component values, the stage around the 4 sec timer and nearby circuitry agrees with the prea section of this 8000A. Other than the pulse starting from a Q13 with its base at a zener 4.7V at switch on and after 4 seconds turning on , will have to see why a jump and not smooth , but design would seem to be for a step change.
As I cannot find anything wrong as far as broken wrong, I will assume this is as designed. I cannot see why there is a low 4K7 dropper from 27V to the
4.7V zener that basically causes this 5V DC pulse, when only supplying 80uA at most into the base of a TO92 transistor, Q13. I will try upping the dropper and adding a C to give a time constant of 2 seconds and change the main hold off time constant from 4 sec to 2 sec also but keeping 13V bias point, both channels. A few strange noises in the first couple of seconds , if that and only if the phones are connected, is surely preferable to 5V DC pulses fed , delayed, into the PA. So both will rise approximately together and so no 5V surge is possible.
That equates to a zener current of around 5mA, which seems an altogether reasonable amount ?
Unless some defined minimum current flows through a zener, then it will not produce a stable and accurate voltage across itself. The current into the base of the transistor is supplementary to the current through the zener, and of itself is generally an insignificant amount, depending on the current demand from the transistor, and the DC gain of that transistor. If you increase the value of the R, then the standing current through the zener will drop, and it will likely not produce the correct voltage across itself. Whilst 5mA through the zener is a 'reasonable' value (assuming it's a little
400mW job), it is never-the-less on the 'low' side of reasonable, and if I was incorporating such a zener into any design of mine, I wouldn't want it to be any lower than that to guarantee correct zener action.
I hear what you're trying to do, but for myself, I'm reluctant to start modifying circuits that I don't fully understand the purpose or functioning of, just because I feel that they may not be working as designed, or even as desired. Sometimes I find that elements of a design turn out to be a 'best effort' compromise by the designer and, even if that circuit element doesn't work quite as he had hoped, it's as good as it can be. Be very careful that any mods you dream up don't make the situation worse. There's no second chances if the wrong amount of DC arrives at the input of a DC coupled amp, as I'm sure you well know ...
Didn't Graham used to be involved with Audiolab gear at some time in the past ? Could be wrong, but it seemed to ring some distant bells in the back of my mind. Where is Graham these days, anyway ? Don't seem to see him on here like we used ...
But DC is arriving at the PA 5V each ch for 0.1s or so every time it is switched on . I cannot make matters worse than that. If you don't have phones connected you never hear the clicks as it is over by the time the relay clicks over for the speaker (normally), so perhaps everyone thinks it is just a matter of the power rails still balancing up rather than "designed" in. Agreed on the zener. I see nothing wrong with putting 47K rather than 4K7 leaving plenty of headroom for functioning reference voltage and base feed. I'm wondering if the designer, whoever it was, had an aversion to putting electrolytic capacitors over zeners. Whenever I come across these Audiolabs now I put a 22uF over each of the 4 , 12V zeners in the pa, as without them amp is liable to hf oscillation (then self-immolation) with large dV/dt spikes fed in the PA input eg fridge click.
Not sure about Audiolab, Graham worked for Studiomaster as a designer, however, these days much of Studiomasters output comes from those 'far eastern' manufacturers these days. (Phonic, SoundKing etc.)
I havent seen or heard from him for quite a while, we used to natter away on MSN. He alluded to a job offer in some far flung part of the planet, maybe he`s just busy.
Well, if the zener does not stabilize the voltage across itself to 4v7 due to the standing current in it being too low after you have 'tweaked' the value of its limiter resistor, but instead goes to some higher voltage, you could ...
If the power amp is not objecting, and you can't actually hear a problem due to the speaker relay not being pulled until after this event anyway, why worry about it? After all, the amp is not really doing anything that it wasn't actually designed to do in the first place. The input drive from the preamp probably reaches 5v peak anyway, so the power amp would not see this as being anything different from a 5v pulse during power up, would it ?
But 47k will reduce the standing current in the zener to around 0.5mA, and with the best will in the world, it ain't gonna produce a stable 4v7 at that low level of current ....
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